search a file for a pattern
-l     (The letter ell.) Write only the names of files containing  selected  lines  to  standard  output.
       Pathnames  shall  be written once per file searched. If the standard input is searched, a pathname
       of "(standard input)" shall be written, in the POSIX locale. In other  locales,  "standard  input"
       may be replaced by something more appropriate in those locales.
source manpages: grep